1. Field of the Invention
The present invention relates to an electron gun used when a three-dimensional (3D) object is fabricated by stacking thin layers of a powdered sample on top of each other on a support stage. The invention also relates to a method of controlling this electron gun. Furthermore, the invention relates to an electron beam additive manufacturing machine.
2. Description of Related Art
There is a widely known additive manufacturing machine that builds a three-dimensional object by spreading resinous powder tightly over the whole support stage to form a powder layer, irradiating the powder layer with laser light to melt the resinous powder, allowing the molten powder to solidify, and stacking layers of such solidified resinous powder on top of each other. In recent years, additive manufacturing machines using an additive layer manufacturing method for fabricating a three-dimensional object by spreading a powder sample tightly over the whole support stage to form a powder layer, irradiating the powder layer with electron beam as used in an electron microscope to melt the powder sample, allowing the molten powder to solidify, and stacking layers of such solidified powder on top of each other have begun to be used. When powdered sample is irradiated with electron beam in this way, electrical current flows through the electron beam. In the following description, this electrical current is referred to as the “beam current”.
In an electron microscope that is one example of instrument using an electron beam, a beam current from on the order of pA to on the order of microamperes is used to observe a tiny sample. The accelerating voltage of an electron gun used in an electron microscope is tens of kV or higher, less than or comparable to hundreds of kV. However, the amount of current is as low as tens of microamperes at most and so the electron beam power is not so large. This permits a beam blocking member to be installed inside the optical system. In an electron microscope, a current-limiting aperture is placed inside the optical system, and a required value of beam current is obtained by controlling the amount of the electron beam passing through the aperture by means of an electromagnetic lens. The advantage of this method of beam current control is that the beam current can be modified while maintaining the electron gun conditions unchanged. That is, the electron gun can continue to be used always under optimum conditions irrespective of beam current conditions.
On the other hand, an electron beam additive manufacturing machine uses a large-output electron beam generally having a large current of tens of mA in order to melt a powdered sample. The beam is accelerated by 10 kV or higher. If such a large-power electron beam irradiates the current-limiting aperture, the aperture may be damaged. This makes it impossible to control the beam current using the current-limiting aperture. If a current-limiting aperture is not used, all of the electron beam emitted from the cathode will reach a powdered sample. As a result, the beam cannot enough be focused. Consequently, it is difficult to fabricate an object accurately. For this reason, in the electron beam additive manufacturing machine, the electron beam emitted from the electron gun is controlled using a bias voltage to vary the beam current. When the bias voltage of the electron gun is varied, what vary concomitantly are not restricted to the beam current. All electron gun characteristics including the brightness and the diameter of the light source vary.
JP-A-1-274349 discloses an electron gun that has a filament emitting thermal electrons, an extractor electrode for extracting thermal electrons from the filament, a Wehnelt electrode for focusing the thermal electrodes extracted from the filament, and an anode for accelerating the focused thermal electrons.
Generally, a process for melting a powdered sample used in an additive manufacturing process starts with preheating the powdered sample. Then, the sample is molten to form fringes of the object to be fabricated. Then, the sample is molten to form the interior of the object. The required value of the beam current of the electron beam irradiated at the powdered sample from the electron gun and the required value of the diameter of the beam when the beam is focused on a Z-axis stage are different for different melting processes. Therefore, it is necessary to control these parameters by varying the operative conditions of the electron gun and of the lens varying the beam diameter. For example, while one powdered layer is being molten, the beam current must be varied from several milliamperes to tens of milliamperes. If the beam current is varied, the brightness of the electron beam changes. This may result in the sample being molten nonuniformly.